1. Field of the Invention
The present invention relates generally to a pulse radar system for determining or measuring a distance to an object of concern (hereinafter also referred to as the target) and more particularly to a pulse radar system for measuring a distance to a target located at a relatively short distance from the radar system.
2. Description of Related Art
A typical one of such radar systems is disclosed in Takashi Yoshida: "RADAR TECHNIQUES" published by the Institute of Electronics, Information and Communication Engineers of Japan, (1984), p. 1. For having better understanding of the invention, background techniques thereof will first be reviewed. FIG. 6 is a schematic block diagram illustrating the principle underlying the distance measurement or range finding operation of a conventional pulse radar system.
Referring to FIG. 6, the pulse radar system (hereinafter also referred to as the radar system) generally denoted by reference numeral 1 includes a transmitter 2 equipped with a transmitting antenna 3a for sending out radio wave pulses having high intensity and short duration toward a target 6, a receiver 4 equipped with a receiving antenna 3b for receiving the echo or radio waves reflected at the target 6, and a signal processor for processing the synchronizing pulse signal supplied from the transmitter 2 upon every sending-out of the radio wave and the output signal from the receiver 4 to thereby arithmetically determine the distance to the target or a relative speed when the radar system and the target are moving relative to each other, angular position of the target or the like which is then displayed on an indicator or a display unit 5 provided at the output side of the signal processor.
The range finding function (i.e., function for measuring the distance to the target) of the radar system will now be considered. When the distance between the radar system 1 and the target 6 is represented by R while representing by T the difference between the time point at which the radio wave was sent out from the transmitting antenna 3a and the time point at which echo from the target 6 (i.e., time lapse from the transmission of the radio wave to the reception of the echo) is received by the receiving antenna 3b, the following expression (1) can apply valid for the distance R and the time difference T. EQU R=CT/2 (1)
where C represents the velocity of light.
Thus, the distance to the target can be arithmetically determined in accordance with the above expression (2) by measuring the time difference, i.e., the time taken for the radio wave transmitted from the radar system to be received by the receiver 4 after reflection at the target. Although the above description is made in conjunction with the pulse radar system, the basic concept underlying the distance measuring or range finding operation of the other type radar systems such as FM-CW radar.
The conventional radar system of the structure described above suffers a problem that when the distance between the radar system 1 and the target 6 is so short that the received signal levels of multiple reflected waves, i.e., radio wave undergone twice or more reflections between the target and the radar system in succession to a first radio wave or echo (hereinafter also referred to as the primary echo) become higher than a minimum reception level set at the receiver 4, then the multiple reflected waves will possibly be recognized or identified as if they originated in other targets, which may of course lead to erroneous detection of the real target, to a serious disadvantage.